The Minimum Temperature
The first question is: how low can you afford to go? The simple answer is 120 degrees Fahrenheit. That’s what the U.S. Department of Energy recommends as the ideal setting, and most new water heaters have this as the preset temperature.
However, keep in mind that setting the temperature too low can turn your water heater into a breeding ground for harmful pathogens. The higher the temperature, the quicker those pathogens will die off. This is the main reason why the OSHA and other agencies recommend a temperature of 140 degrees Fahrenheit.
For most people, 120 degrees should be safe enough. That said, having someone with a compromised immune system in your family would be a good reason to set the temperature a bit higher. The same applies if you have a dishwasher that doesn’t produce its own heat.
At 140 degrees, water is capable of causing a third-degree burn in five seconds. Hot water also needs a few seconds to travel through the pipes, so you’ll never be quite sure when it’ll get to your faucet. Scalding water is particularly dangerous to kids (due to their thinner skin) and the elderly (on the account of reduced mobility, sensitivity, and reaction time).
One solution that protects you from both scalding and pathogenic bacteria is the anti-scald valve. By installing these valves at contact points with hot water, you can set your water heater to 140 degrees while knowing that the water at the tap will never get hotter than 120 degrees.